Windows 10: New PC, keep rebooting after inserting product key

Hello there,
I have a new computer with windows 10, at the first boot the computer did some things to prepare itself for use, but then I was asked to give my credenitals to account, I used my windows account, but there was some error and I had to reboot.

Since then this is what happens:
when I boot up there is (probably, cause English is not my native language)
Hi there
let's ́s get a few basic things out of the way

Then I choose my language and click next, after that it asks for a product key, after I confirm that (or skip) and use express settings it goes into "just a moment" than the computer reboot and I am at the begining again:
Hi there
let's ́s get a few basic things out of the way

:)

 

Installing windows 7 while running windows 10 for over 30 days.

I upgraded my PC to windows 10 quite a few months back on my Dell Optiplex 780 pc. Since then, the pc keeps rebooting when starting up and automatically keeps rebooting at random times. I want to go back to windows 7, the original os of my pc, to see if
it would solve this problem. However, I couldn't do it through settings because I have had windows 10 for many months now and I can't revert back to windows 7. When I try to do it through the Microsoft website, it gives me the error message below after I
put in my product key. I was wondering if there is any solution to this problem without having to buy a new product key.

Error
The product key you entered appears to be for software pre-installed by the device manufacturer. Please contact the device manufacturer for software recovery options.

P.S. If you know how to fix that rebooting problem, that would be even better.

 

lost 10 Pro key

Did Pro come with the old PC, or did you upgrade to it from Store or using Product Key? If Store then it gave permanent entitlement to the old PC only for Pro version. If Product Key you might be able to insert it in new PC Activation settings to Upgrade
to Pro but the old PC could no longer use Pro.

 

Return it back to where you bought it and have the guys fix it.

 

I´ve managed to solve myself, turned on support for windows 8/8.1 in BIOS and after next reboot it showed that Windows failed to boot and I finally had a chance to return machine to the factory setting. That helped since it installed windows again and all went well as should on the first try.